ASSOCIATION OF TYROSINE KINASE P56(LCK) WITH CD4 INHIBITS THE INDUCTION OF GROWTH THROUGH THE ALPHA-BETA T-CELL RECEPTOR

THE membrane glycoprotein CD4 enhances antigen-mediated activation of T cells restricted by class II molecules of the major histocompatibility complex (MHC)1-3. This positive function has been attributed to the protein tyrosine kinase p56lck (ref. 4), which is noncovalently associated with the cytoplasmic portion of CD45,6, and is activated on CD4 aggregation7. Antigen presentation by MHC class II molecules coaggregates CD4 and the T-cell antigen receptor (TCR-alpha-beta-CD3)8. Thus, the mutual specificity of CD4 and TCR-alpha-beta for the MHC-antigen complex results in the juxtaposition of p56lck and TCRalpha-beta-CD39-13. In contrast, antibodies can abrogate antigen-induced14-17, as well as anti induced18-20 T-cell activation, indicating that CD4 might also transduce negative signals. The molecular basis for this opposing function remains unclear. Here we show that the CD4-p56lck complex prohibits the induction of activation signals through the TCR-CD3 complex when not specifically included in the signalling process. This negative effect does not require anti-CD4 treatment, indicating that the induction of distinct negative signals is probably not involved. Rather, the results demonstrate that the CD4-p56lck complex provides prerequisite signals for antigen-receptor-induced T-cell growth and thus characterize a molecular mechanism for functional constraints imposed on T-cell activation by the MHC.